Abstract

Background

Many previous studies have documented seasonal variation in suicides globally. We
re-assessed the seasonal variation of suicides in Finland and tried to relate it to
the seasonal variation in daylength and ambient temperature and in the discrepancy
between local time and solar time.

Methods

The daily data of all suicides from 1969 to 2003 in Finland (N = 43,393) were
available. The calendar year was divided into twelve periods according to the length
of daylight and the routinely changing time difference between sun time and official
time. The daily mean of suicide mortality was calculated for each of these periods and
the 95% confidence intervals of the daily means were used to evaluate the statistical
significance of the means. In addition, daily changes in sunshine hours and mean
temperature were compared to the daily means of suicide mortality in two locations
during these afore mentioned periods.

Results

A significant peak of the daily mean value of suicide mortality occurred in Finland
between May 15th and July 25th, a period that lies symmetrically around the solstice.
Concerning the suicide mortality among men in the northern location (Oulu), the peak
was postponed as compared with the southern location (Helsinki). The daily variation
in temperature or in sunshine did not have significant association with suicide
mortality in these two locations.

Conclusions

The period with the longest length of the day associated with the increased suicide
mortality. Furthermore, since the peak of suicide mortality seems to manifest later
during the year in the north, some other physical or biological signals, besides the
variation in daylight, may be involved. In order to have novel means for suicide
prevention, the assessment of susceptibility to the circadian misalignment might
help.

Background

Current data on the routinely occurring peaks of deaths from suicide are conflicting
[1,2].
However, for the past four decades in Finland, the seasonal pattern has been stronger the
lower the suicide mortality has been [3]. There is a
clear peak of suicide occurrence around May or June [4-7] and a preceding peak in suicide
attempts around April [8]. Furthermore, another
smaller peak of suicide occurrence exists around October [7,9]. These two mortality peaks, being
similar and more robust the further away the country locates from the equator, have been
explained by socio-demographic and socio-economic factors [10], but since this seasonal pattern has existed for decades [11], if not centuries [12], biological factors are likely.

Major depressive episodes are known to contribute to suicide substantially [13,14], and a
history of mood disorders and psychiatric hospitalization associates clearly with the
seasonal occurrence of suicides [15,16]. Desynchronization of physiological rhythms, e.g.
desynchronization of the circadian rhythm of core body temperature with the sleep-wake
cycle [17-19] and some clock gene variants [20,21], can be associated with mood disorders. Based on
our earlier psychological autopsy studies of death from suicide [22] and the data from the nationwide suicide program in Finland [23], we hypothesized that the circadian misalignment
among the depressed may increase during spring, and thereby predispose to suicidal
behaviors [24].

Rest-activity cycles during the day [25] and sleep
stages at night [26] are controlled by circadian
clocks, but they are frequently disturbed among the depressed. Furthermore, the principal
circadian clock entrains to the sun light [27-29], by tracking the daily changes in rise and set
times of the sun and the variation in the length of the day [30-32]. Thus, the timing of
light exposure is relevant to entrainment and influences the course of mood disorders
[33,34].
Therefore, we hypothesized that it is the key to the suicide mortality peaks whether the
light-dark transitions give the principal circadian pacemaker a signal to accelerate or
decelerate, especially among the depressed. In addition, since sunshine and ambient
temperature are potential time-givers, modulate the function of biological clocks [35], and associate with deaths from suicide [3,36], we aimed
to test their effect, as well.

Methods

Statistics Finland http://www.stat.fi provided us with the daily data of 43,393 suicides,
33,993 of men and 9400 (22%) of women, committed in Finland during the 35-year period of
1969 to 2003 (Tables 1 and 2). Two phenomena, which affect the timing and the speed of the
light-dark transitions regularly each year, were selected a priori as the potential
factors that might challenge the biological clocks and produce circadian misalignment.
First, we focused on the length of the photoperiod, because at high to temperate latitudes
around spring and fall equinoxes the transitions between day and night are most rapid and
the durations of twilight short, as a consequence of the rotation of the earth. Second, we
focused on the constant mismatch between the sun time (hereafter ST) and the coordinated
universal time (hereafter UCT), arising from the earth's tilt and elliptical orbit around
the sun.

Oulu

Helsinki

Finland

Year

Men

S

per100000

Men

S

per100000

Men

S

per100000

1969

42 447

11

25.92

272 321

110

40.39

2 230 217

850

38.11

1970

41 412

13

31.39

266 174

108

40.58

2 219 985

763

34.37

1971

42 798

11

25.70

271 393

117

43.11

2 234 037

781

34.96

1972

43 436

22

50.65

275 378

132

47.93

2 249 051

874

38.86

1973

44 127

27

61.19

277 205

109

39.32

2 262 142

849

37.53

1974

45 082

22

48.80

278 485

131

47.04

2 273 815

921

40.51

1975

45 815

22

48.02

278 628

128

45.40

2 282 115

924

40.49

1976

46 069

29

62.95

278 693

152

54.54

2 286 392

967

42.29

1977

46 444

22

47.37

277 978

154

55.40

2 295 668

962

41.91

1978

46 609

13

27.89

277 735

156

56.17

2 300 790

963

41.86

1979

46 533

18

38.68

278 569

133

47.74

2 306 784

935

40.53

1980

46 779

24

51.31

279 456

145

51.89

2 314 843

962

41.56

1981

47 343

21

44.36

280 580

151

53.82

2 327 473

904

38.84

1982

48 179

18

37.36

282 751

134

47.39

2 342 869

905

38.63

1983

48 331

25

51.73

284 565

130

45.68

2 357 172

938

39.79

1984

48 620

25

51.42

286 092

149

52.08

2 369 228

988

41.70

1985

49 065

23

46.88

287 858

113

39.26

2 377 780

964

40.54

1986

49 405

30

60.72

290 370

149

51.31

2 385 866

1023

42.88

1987

49 890

28

56.12

292 935

137

46.77

2 392 868

1068

44.63

1988

50 138

44

87.76

294 242

150

50.98

2 401 368

1112

46.31

1989

50 951

29

56.92

295 665

160

54.12

2 412 760

1121

46.46

1990

51 623

33

63.93

298 420

198

66.35

2 426 204

1199

49.42

1991

52 254

35

66.98

302 609

185

61.14

2 443 042

1193

48.83

1992

52 959

36

67.98

306 298

204

66.60

2 457 282

1160

47.21

1993

53 495

35

65.43

311 134

172

55.28

2 470 196

1112

45.02

1994

54 661

23

42.08

316 367

176

55.63

2 481 649

1080

43.52

1995

56 132

26

46.32

322 074

179

55.58

2 491 701

1081

43.38

1996

57 436

26

45.27

327 168

131

40.04

2 500 596

966

38.63

1997

58 482

36

61.56

332 113

158

47.57

2 509 098

1039

41.41

1998

59 606

26

43.62

337 297

121

35.87

2 516 075

965

38.35

1999

61 025

40

65.55

341 125

139

40.75

2 523 026

961

38.09

2000

62 800

28

44.59

344 520

143

41.51

2 529 341

879

34.75

2001

64 116

31

48.35

347 925

150

43.11

2 537 597

936

36.89

2002

64 995

22

33.85

349 121

139

39.81

2 544 916

825

32.42

2003

65 965

29

43.96

350 334

119

33.97

2 552 893

823

32.24

Table 1

Men's suicides in numbers during the study period

The yearly male population, number of suicides(S), and suicide mortality for men in
Oulu, Helsinki, and Finland from 1969 to 2003.

Oulu

Helsinki

Finland

Year

Women

S

per100000

Women

S

per100000

Women

S

per100000

1969

46 245

5

10.81

333 502

43

12.89

2 384 060

246

10.32

1970

45 656

4

8.76

325 034

58

17.84

2 378 351

220

9.25

1971

46 474

5

10.76

330 205

58

17.57

2 391 875

222

9.28

1972

47 633

5

10.50

333 507

57

17.09

2 404 350

239

9.94

1973

48 302

6

12.42

335 687

60

17.87

2 416 619

249

10.30

1974

49 272

11

22.33

337 470

47

13.93

2 428 572

255

10.50

1975

50 132

5

9.97

337 570

72

21.33

2 438 377

254

10.42

1976

50 410

12

23.81

336 980

66

19.59

2 444 444

253

10.35

1977

50 691

8

15.78

335 057

54

16.12

2 451 299

258

10.53

1978

50 964

11

21.58

334 547

53

15.84

2 457 298

237

9.65

1979

51 188

6

11.72

334 981

60

17.91

2 464 508

242

9.82

1980

51 582

4

7.76

335 630

63

18.77

2 472 935

264

10.68

1981

52 237

1

1.91

336 511

56

16.64

2 484 677

239

9.62

1982

53 059

10

18.85

338 116

59

17.45

2 498 846

267

10.69

1983

53 225

6

11.27

339 108

56

16.51

2 512 686

249

9.91

1984

53 443

3

5.61

340 162

54

15.88

2 524 520

253

10.02

1985

53 976

4

7.41

341 781

58

16.97

2 532 884

249

9.83

1986

54 349

12

22.08

343 576

53

15.43

2 539 778

287

11.30

1987

54 760

10

18.26

346 162

65

18.78

2 545 734

301

11.82

1988

55 125

9

16.33

346 880

62

17.87

2 552 991

296

11.59

1989

55 810

8

14.33

347 226

58

16.70

2 561 623

297

12.59

1990

56 294

8

14.21

348 913

90

25.79

2 572 274

324

11.60

1991

56 735

8

14.10

352 207

74

21.01

2 585 960

306

11.83

1992

57 391

6

10.46

354 429

77

21.73

2 597 700

297

11.43

1993

57 765

6

10.39

358 557

72

20.08

2 607 716

293

11.24

1994

58 781

12

20.42

363 774

77

21.17

2 617 105

307

11.73

1995

60 186

12

19.94

369 437

65

17.59

2 625 125

309

11.77

1996

61 447

8

13.02

373 663

69

18.47

2 631 724

282

10.72

1997

62 439

11

17.62

378 547

68

17.96

2 638 251

284

10.77

1998

63 454

11

17.34

382 880

57

14.89

2 643 571

268

10.14

1999

64 516

13

20.15

386 384

68

17.60

2 648 276

254

9.59

2000

66 149

11

16.63

389 425

66

16.95

2 651 774

292

11.01

2001

67 584

9

13.32

391 649

62

15.83

2 657 304

271

10.20

2002

68 297

11

16.11

392 485

48

12.23

2 661 379

275

10.33

2003

68 878

7

10.16

393 035

55

13.99

2 666 839

261

9.79

Table 2

Women's suicides in numbers during the study period

The yearly female population, number of suicides(S), and suicide mortality for women
in Oulu, Helsinki, and Finland from 1969 to 2003.

The nominal calendar year was split into twelve periods according to these two phenomena;
first into four astronomical seasons, which are determined by spring and fall equinoxes
and summer and winter solstices (for the definition, see http://asa.nao.rl.ac.uk/), and second
into eight periods, by the equation of time (for the definition, see http://www.nmm.ac.uk/explore/astronomy-and-time/time-facts/the-equation-of-time),
as follows (see also Figure 1). From February
11th to May 14th (hereafter marked as X1) and from July 26th
to November 3rd (X2) ST goes fast compared with UCT and in between those
periods, that is, from May 15th to July 25th (Y1) and from November
4th to February 10th (Y2) it goes slow. Furthermore, another
categorization was made based on the equation of time separating periods when ST is either
ahead or behind the UCT. In other words, ST is constantly ahead of the UCT, from April
15th to June 13th (A1) and from September 1st to
December 25th (A2), and constantly behind the UCT, from June 14th to
August 31st (D1), and from December 26th to April 14th
(D2). Hence, ST deviates from UCT constantly and is maximally behind at February
11th (approximately 14 minutes) and vice versa maximally ahead at November
3rd (approximately 16 minutes). The Almanac Office at the University of
Helsinki http://almanakka.helsinki.fi/ both provided the dates for the astronomical
seasons and calculated the dates for the periods (X1, Y1, X2, Y2) of the equation of time,
as well as the dates for the periods (A1, D1, A2, D2) through the whole study period.

Figure 1

Periods according to time of equation and astronomical seasons. During
X1(February 11-May 04) and X2 (July 26-November 03) (marked with pink lines) sun time
is accelerating, and during Y1 (May 15-July25) and Y2 (November 04-February 10)
(marked with green lines) it is decelerating compared with the coordinated universal
time (UCT). During A1 (April 15-June13) and A2 (September 01-December 25) sun time
stays ahead and during D1 (June 14-August 31) and D2 (December 26-April 14) it stays
behind the UCT. Astronomical seasons are separated with dotted vertical lines. During
astronomical spring and summer daylight exceeds darkness, and vice versa during
astronomical fall and winter darkness exceeds daylight in Finland. Y-axis on the left
side presents the time difference (in minutes) that sun time deviates from the
UCT.

To evaluate the effect of daily sunshine hours and temperature on suicide mortality, we
focused on two cities on a similar longitude but with dissimilar photoperiod: first,
Helsinki (60°9.7'N, 24°57.3'E), which is the capital of Finland in the south,
and second, Oulu (65°1.0'N, 25°30.0'E), which is a central city of the northern
part of the country, 600 km north from Helsinki. In Helsinki 5062 suicides were committed
by men, and 2160 by women, whereas 903 by men and 278 by women in Oulu. The Finnish
Meteorological Institute http://www.fmi.fi/ provided us with the daily data on sunshine hours and
temperature, measured within the 25-km radius from these cities throughout the study
period. For the day to day analysis, the daily sunshine, measured in minutes per day,
(hereafter S) and the daily temperature, measured in degrees in Celsius and averaged as
the daily mean value, (hereafter T) were compared with those on the previous day and
changes were marked as (+) indicating an increase, and (-) indicating a decrease from the
previous day. Thus, we ended with four types of days according to weather changes, coded
as T+S+, T+S-, T-S+, and T-S-, concerning the data from Helsinki and Oulu regions.

In order to take into account the differences in the yearly population sizes within and
between Helsinki and Oulu, the daily means of suicides were calculated into daily means of
suicide mortality rates (suicides per 100,000), for men and women, per each year, and for
both cities (Tables 1 and 2). Furthermore, in order to control for the different lengths of each
period studied, and to avoid the bias of having dominance of certain type of weather
changes within any period of the year, the daily mean of suicide mortalities (number of
suicides per day, with 95% confidence intervals [CIs]) was calculated for each period in
separate (Tables 3, 4, and 5).

Area

Selected Days

Astronomical season

Winter

Spring

Summer

Fall

M

Finland

All

.099.094-.103

.120.115-.125

.117.112-.122

.106.101-.111

Helsinki

All

.128.120-.136

.138.128-.148

.135.123-.147

.131.122-.140

T+

S+

.123.099-.146

.135.122-.147

.139.122-.156

.137.120-.155

S-

.113.092-.134

.144.125-.163

.142.120-.164

.115.099-.132

T-

S+

.122.108-.136

.146.128-.163

.130.117-.143

.133.118-.147

S-

.122.097-.146

.140.120-.161

.129.112-.146

.144.116-.172

Oulu

All

.119.098-.140

.141.123-.160

.159.138-.180

.131.113-.150

T+

S+

.107.050-.163

.134.103-.165

.162.112-.212

.152.077-.227

S-

.097.059-.135

.126.082-.170

.175.132-.217

.147.101-.193

T-

S+

.124.094-.154

.167.132-.203

.156.115-.197

.117.073-.161

S-

.097.045-.148

.138.110-.166

.132.086-.178

.144.083-.205

W

Finland

All

.025.023-.027

.031.030-.032

.029.028-.031

.028.027-030

Helsinki

All

.046.042-.050

.050.045-.054

.049.045-.054

.048.043-.053

T+

S+

.038.028-.048

.056.046-.065

.049.039-.058

.056.043-.069

S-

.051.033-.069

.049.041-.056

.046.034-.058

.052.042-.062

T-

S+

.042.036-.049

.046.034-.057

.049.040-.057

.049.039-.059

S-

.052.039-.065

.052.042-.062

.051.041-.061

.045.031-.059

Oulu

All

.036.029-.044

.040.030-.049

.042.032-.051

.039.029-.048

T+

S+

.032.006-.058

.031.016-.046

.042.023-.061

.049.013-.086

S-

.037.015-.058

.030.015-.043

.047.020-.074

.020.005-.036

T-

S+

.041.019-.063

.051.028-.074

.037.019-.055

.043.019-.067

S-

.028.000-.055

.045.020-.069

.042.020-.063

.047.018-.076

Table 3

Astronomical seasons and men's (M) and women's (W) suicide mortality

Daily mean of suicide mortality and confidence interval of the mean in aggregate over
the years from 1969 to 2003 for men and women, during astronomical seasons in Finland,
Helsinki and Oulu, and according to daily changes (+/-) in temperature (T) and
sunshine hours(S) in Helsinki and Oulu.

Winter: 21.12-20.03

Spring: 21.03-20.06

Summer: 21.06-22.09

Fall: 23.09-20.12

Area

Selected Days

Periods of time of equation

X1

Y1

X2

Y2

M

Finland

All

.109.105-.114

.124.118-.129

.112.107-.116

.101.096-.106

Helsinki

All

.131.121-.141

.142.130-.154

.130.122-.139

.130.120-.140

T+

S+

.139.116-.161

.149.131-.167

.122.097-.147

.124.097-.151

S-

.151.123-.179

.145.119-.170

.124.103-.145

.110.088-.131

T-

S+

.152.122-.182

.126.100-.153

.135.121-.150

.120.099-.141

S-

.133.104-.161

.124.106-.142

.138.116-.160

.114.086-.142

Oulu

All

.133.115-.150

.146.124-.167

.150.131-.169

.122104-.140

T+

S+

.156.094-.218

.149.101-.198

.206.122-.290

.131.052-.210

S-

.096.042-.150

.207.142-.272

.165.112-.217

.097.046-.148

T-

S+

.199.138-.259

.140.090-.190

.138.092-.184

.115.068-.162

S-

.123.065-.181

.158.114-.201

.175.107-.242

.056.008-.103

W

Finland

All

.028.027-.029

.032.030-.033

.030.029-.031

.025.024-.027

Helsinki

All

.047.043-.051

.053.048-.058

.050.045-.054

.043.039-.048

T+

S+

.052.040-.064

.055.046-.065

.056.044-.068

.033.018-.048

S-

.044.030-.057

.059.047-.072

.055.041-.069

.055.044-.067

T-

S+

.051.032-.070

.049.036-.061

.049.040-.058

.036.028-.045

S-

.051.034-.067

.046.034-.058

.048.037-.058

.045.025-.065

Oulu

All

.038.029-.047

.045.034-.057

.035.027-.042

.038.029-.047

T+

S+S-

.023.005-.042

.039.017-.060

.044.014-.074

-

.035.012-.058

.053.018-.087

.015.001-.029

.026.002-.050

T-

S+S-

.062.028-.096

.030.006-.054

.042.024-.061

.036.013-.059

.030.003-.058

.060028-.091

.032.010-.054

-

Table 4

Accelerating and decelerating periods of the equation of time and men's (M) and
women's (W) suicide mortality

Daily mean of suicide mortality and confidence interval of the mean in aggregate over
the years from 1969 to 2003 for men and women, during accelerating (X1, X2) and
decelerating (Y1, Y2) periods of the equation of time in Finland, Helsinki and Oulu,
and according to daily changes (+/-) in temperature (T) and sunshine hours(S) in
Helsinki and Oulu.

X1: 11.02-14.05

Y1: 15.05-25.07

X2: 26.07-03.11

Y2: 04.11-10.2

Area

Selected days

Periods of time of equation

D2

A1

D1

A2

M

Finland

All

.102.098-.107

.125.120-.130

.118.112-.123

.107.102-.111

Helsinki

All

.128.121-.136

.146.134-.158

.135.122-.147

.130.122-.139

T+

S+

.125.103-.146

.142.129-.154

.141.123-.159

.128.108-.148

S-

.123.102-.143

.142.119-.164

.145.121-.169

.116.100-.131

T-

S+

.124.111-.136

.162.137-.187

.124.108-.141

.134.121-.146

S-

.123.103-.144

.147.124-.171

.126.110-.142

.141.118-.164

Oulu

All

.125.107-.144

.145.123-.167

.148.128-.168

.139.119-.159

T+

S+

.099.059-.139

.159.123-.196

.137.088-.185

.186.121-.250

S-

.105.065-.145

.129.080-.178

.177.129-.225

.150.104-.196

T-

S+

.140.112-.168

.155.103-.208

.146.095-.197

.129.090-.168

S-

.109.062-.157

.137.097-.176

.122.079-.165

.149.091-.206

W

Finland

All

.026.024-.027

.032.030-.034

.030.028-.031

.028.027-.030

Helsinki

All

.046.042-.049

.051.045-.056

.051.046-.056

.048.043-.053

T+

S+

.043.034-.053

.058.047-.069

.048.039-.057

.056.047-.065

S-

.050.033-.067

.047.033-.062

.049.038-.061

.052.042-.062

T-

S+

.041.034-.048

.051.036-.066

.050.040-.061

.048.040-.056

S-

.048.039-.058

.057.044-.069

.051.040-.062

.048.037-.059

Oulu

All

.036.029-.044

.043.032-.054

.041.030-.053

.038.031-.048

T+

S+

.037.016-.059

.031.013-.050

.036.017-.054

.044.019-.069

S-

.033.016-.049

.033.013-.054

.058.024-.092

.017.005-.030

T-

S+

.045.026-.063

.047.020-.075

.040.014-.066

.040.022-.058

S-

.025.002-.048

.053.021-.085

.041.019-.064

.039.020-.057

Table 5

Advanced and delayed periods of the equation of time and men's (M) and women's (W)
suicide mortality

Daily mean of suicide mortality and confidence interval of the mean, in aggregate
over the years from 1969 to 2003 for men and women, during advanced (A1, A2) and
delayed (D1, D2) periods of the equation of time in Finland, Helsinki and Oulu, and
according to daily changes (+/-) in temperature (T) and sunshine hours(S) in Helsinki
and Oulu.

A1: 15.04-13.06

D1: 14.06-31.08

A2: 01.09-25.12

D2: 26.12-14.04

Finally, to rule out a potential confounder, we analyzed whether daylight saving time
(hereafter DST) had any effect on the suicide mortality. DST was introduced in Finland
1981. From 1981 to 1994 DST lasted from the end of March until the end of September
(hereafter DST1), and since 1995 DST has been in use from the end of March until the end
of October, as in most parts of Europe (hereafter DST2). We calculated suicide mortality
rates during one month period before, and after the transitions into and out of DST,
separately for the years 1981 to 1994 (DST1) and years 1995 to 2003 (DST2), for which the
suicide mortality rates of the corresponding periods during the years 1969 to 1980 were
used as controls (Tables 6, 7, 8, 9,10, and 11).

Days

1969-80*

1981-2003

-1 m

+1 m

-1 m

+1 m

M

CI

m

CI

m

CI

m

CI

Finland

All

.097

.089-.106

.111

.102-.121

.104

.097-.111

.117

.110-.124

Helsinki

All

.114

.096-.133

.130

.114-.146

.135

.120-.150

.137

.118-.155

T+S+

.120

.061-.180

.160

.100-.219

.129

.084-.173

.140

.102-.177

T+S-

.100

.054-.146

.156

.100-.212

.128

.084-.171

.181

.143-.220

T-S+

.123

.090-.156

.102

.067-.137

.139

.108-.171

.142

.100-.184

T-S-

.144

.044-.243

.127

.061-.194

.180

.114-.246

.108

.080-.136

Oulu

All

.113

.064-.162

.114

.060-.168

.121

.092-.150

.158

.117-.199

T+S+

-

-

-

-

-

-

.110

.042-.179

T+S-

-

-

-

-

.145

.060-.229

.146

.055-.237

T-S+

.123

.037-.208

.152

.013-.291

.104

.038-.171

.21

.139-.280

T-S-

-

-

-

-

.065

.002-.129

.138

.039-.237

Table 6

Men: Daily mean of suicide mortality, and switching into daylight saving time in
spring.

Daily mean of suicide mortality during one month period before (-1 m), and after (+1
m) the switch into daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Location

Days

1969-80 *

1981-2003(DST1 + DST2)

-1 m

+1 m

-1 m

+1 m

m

CI

m

CI

m

CI

m

CI

Finland

All

.024

.020-.028

.026

.023-.029

.027

.024-.030

.030

.027-.033

Helsinki

All

.055

.048-.062

.043

.031-.055

.043

.034-.052

.049

.040-.059

T+S+

.042

.014-.071

.052

.020-.083

.047

.019-.076

.055

.035-.076

T+S-

.054

.028-.080

.031

.010-.052

.024

.012-.037

.046

.030-.062

T-S+

.061

.041-.081

.029

.004-.053

.040

.022-.057

.065

.028-.101

T-S-

.052

.018-.087

-

-

.049

.020-.079

.044

.024-.065

Oulu

All

.033

.006-.059

.048

.016-.080

.048

.030-.066

.030

.009-.050

T+S+

-

-

-

-

-

-

-

-

T+S-

-

-

-

-

.053

.013-.094

-

-

T-S+

-

-

-

-

.067

.018-.117

-

-

T-S-

-

-

-

-

-

-

-

-

Table 7

Women: Daily mean of suicide mortality and switching into daylight saving time in
spring.

Daily mean of suicide mortality during one month period before (-1 m), and after (+1
m) the switch into daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Location

Days

1969-80*

1981-94 (DST1)

-1 m

+1 m

-1 m

+1 m

m

CI

m

CI

m

CI

m

CI

Finland

All

.111

.099-.122

.101

.094-.108

.119

.109-.128

.120

.112-.128

Helsinki

All

.141

.106-.177

.120

.104-.137

.139

.125-.153

.140

.122-.158

T+S+

.151

.064-.238

.115

.078-.151

.091

.039-.143

.163

.107-.219

T+S-

.156

.076-.236

.107

.048-.166

.124

.087-.161

.123

.079-.167

T-S+

.137

.089-.185

.132

.083-.182

.156

.115-.197

.144

.107-.180

T-S-

.138

.087-.189

.147

.085-.208

.184

.125-.244

.155

.062-.248

Oulu

All

.114

.060-.168

.125

.092-.158

.239

.165-.313

.121

.067-.175

T+S+

-

-

-

-

.324

.113-.535

-

-

T+S-

.100

.001-.200

.107

.002-.211

.256

.053-.458

.110

.000-.220

T-S+

.189

.084-.295

.130

.024-.237

.182

.060-.303

.103

.029-.176

T-S-

-

-

-

-

.267

.142-.392

.229

.065-.393

Table 8

Men: Daily mean of suicide mortality, and switching away from daylight saving time in
fall.

Daily mean of suicide mortality during one month period before (-1 m), and after (+1
m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Location

Days

1969-80 *

1981-94 (DST1)

-1 m

+1 m

-1 m

+1 m

m

CI

m

CI

m

CI

m

CI

Finland

All

.028

.025-.032

.029

.026-.033

.029

.024-.033

.035

.031-.039

Helsinki

All

.052

.039-.064

.048

.033-.062

.046

.030-.061

.064

.049-.078

T+

S+

.071

.045-.097

.039

.019-.058

-

-

.074

.040-.108

S-

-

-

.069

.033-.104

.049

.015-.084

.058

.033-.083

T-

S+

.049

.025-.073

.055

.021-.089

.052

.031-.073

.067

.038-.096

S-

.070

.022-.118

.023

.001-.046

.032

.005-.059

.065

.026-.103

Oulu

All

.033

.011-.055

.043

.006-.081

.020

.001-.040

.024

.001-.047

T+S+

-

-

-

-

-

-

-

-

T+S-

-

-

-

-

-

-

-

-

T-S+

-

-

-

-

-

-

-

-

T-S-

-

-

-

-

-

-

-

-

Table 9

Women: Daily mean of suicide mortality, and switching away from daylight saving time
in fall.

Daily mean of suicide mortality during one month period before (-1 m), and after (+1
m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Location

Days

1969-80*

1995-2003 (DST2)

-1 m

+1 m

-1 m

+1 m

m

CI

m

CI

m

CI

m

CI

Finland

All

.101

.094-.108

.101

.093-.110

.105

.095-.115

.097

.088-.105

Helsinki

All

.126

.108-.145

.140

.119-.161

.118

.079-.157

.106

.090-.121

T+S+

.116

.073-.159

.181

.078-.285

.128

.045-.212

.080

.033-.128

T+S-

.134

.085-.182

.072

.017-.127

.117

.059-.176

.109

.075-.142

T-S+

.135

.079-.191

.177

.105-.249

.112

.071-.153

.093

.054-.131

T-S-

.152

.072-.233

.204

.093-.314

.099

.035-.163

.104

.066-.142

Oulu

All

.131

.089-.173

.106

.058-.154

.145

.096-.195

.169

.107-.230

T+S+

-

-

-

-

-

-

-

-

T+S-

.104

.003-.205

-

-

.200

.069-.332

-

-

T-S+

.167

.050-.284

.085

.001-.169

-

-

-

-

T-S-

-

-

-

-

.148

.006-.290

-

-

Table 10

Men: Daily mean of suicide mortality, and switching away from daylight saving time in
fall.

Daily mean of suicide mortality during one month period before (-1 m), and after (+1
m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

Location

Days

1969-80 *

1995-2003 (DST2)

-1 m

+1 m

-1 m

+1 m

m

CI

m

CI

m

CI

m

CI

Finland

All

.029

.026-.032

.025

.022-.029

.032

.026-.038

.027

.022-.032

Helsinki

All

.044

.029-.058

.042

.033-.051

.054

.039-.070

.035

.026-.044

T+S+

.032

.011-.052

.058

.019-.097

.104

.050-.159

.053

.002-.104

T+S-

.055

.027-.083

.074

.036-.112

.049

.015-.083

-

-

T-S+

.046

.015-.076

.032

.010-.054

.034

.002-.067

.040

.016-.065

T-S-

.026

.001-.051

-

-

.064

.030-.099

-

-

Oulu

All

.038

.005-.072

.032

.010-.054

.027

.004-.051

.051

.012-.090

T+S+

-

-

-

-

-

-

-

-

T+S-

-

-

-

-

-

-

-

-

T-S+

-

-

-

-

-

-

-

-

T-S-

-

-

-

-

-

-

-

-

Table 11

Women: Daily mean of suicide mortality, and switching away from daylight saving time
in fall.

Daily mean of suicide mortality during one month period before (-1 m), and after (+1
m) the switch away daylight saving time.

* = DST was not in use in Finland

m = daily mean of suicide mortality

CI = confidence interval of the mean

The 95% CIs of the daily mean values, controlled for the length of a period of study and
the male and female population sizes in a region of study, were used to evaluate the
statistical significance, so that if they did not overlap with each other, it was judged
to indicate a marked statistical significance.

Results

In Finland, during the years 1969 to 2003, the daily mean of suicide mortality was at the
highest, with a statistical significance, for both men (mean = .124, CI = .118-.129) and
women (mean = .032, CI = .030-.033), during the period Y1, i.e. from May 14th
to July 25th , as compared to the nationwide references (Table 4).

Local photoperiod

The highest daily mean of suicide mortality seem to have emerged later in Oulu compared
with Helsinki, but only for men. Therefore, the results of men are reported here in more
detail. The daily mean of suicide mortality was at the highest during the period Y1 in
Helsinki (mean = .142, CI = .130-.154, Table 4),
but during the period X2 i.e. from July 26th to November 3rd in
Oulu (mean = .150, CI = .131-.169, Table 4). The
same postponed pattern was found also when the time pattern of suicide mortality was
evaluated by seasons. The daily mean of suicide mortality was highest in Helsinki during
spring (mean = .138, CI = .128-.148), but during summer in Oulu (mean = .159, CI =
.138-.180). Furthermore, a similar postponed pattern was seen from A1 (Helsinki) to D1
(Oulu) periods (Table 5). However, these results
did not reach statistical significance.

Local daily weather changes

For men, the days with T+S+ seem to have had the highest daily mean of suicide
mortality both in Helsinki, during the period Y1 (mean = .149, CI = .131-.167), and in
Oulu, during the period X2 (mean = .206, CI = .122-.290), which were the most
"dangerous" periods in these cities. However, when estimated by the 95% confidence
intervals, there was no statistical difference in the variation of means of suicide
mortality between the four types of weather changes. The daily mean of suicide mortality
in Helsinki and Oulu, however, do exceed the nationwide daily means of suicide mortality
(mean = .124 for Y1 in Finland, and mean = .112 for X2 in Finland), as do all the
underlined values for different types of weather changes in Helsinki and Oulu compared
with each period at issue in Tables 3, 4, and 5.

Daylight saving time

The use or timing of daylight saving time did not have a significant effect on the
suicide mortality (Tables 6, 7, 8, 9, 10, and
11).

Discussion

Nationwide results

Our key finding of statistical significance demonstrates the increased suicide
mortality on nationwide level in Finland during the period from May 14th to
July 25th. This 76-day period covers symmetrically both sides of summer
solstice (Figure 1). During this period there is
only 1 to 4 hours of darkness during the night in Helsinki but no darkness at all in
Oulu. For the photoperiod dynamics in these locations, see http://www.gaisma.com/en/location/helsinki.html and http://www.gaisma.com/en/location/oulu.html, whose sunrise and sunset
calculations are based on the algorithms displayed on
National Oceanic and Atmospheric Administration Surface Radiation Research
Branch web site at http://esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html, and e.g. for
latitudes less than 72° north and south, accuracy is approximately one minute. It
is of note here that the photoperiod in Finland due to its time zone is asymmetrical
throughout the year, the period of daylight being always shorter for the a.m. hours than
it is for the p.m. hours. This phenomenon influences the mechanisms that decode the
duration of the melatonin signal in the melatonin-target tissues.

From the circadian-clock point of view, this period (May 14th to July
25th) is a challenge to alignment of the circadian rhythms with the
sleep-wake cycle, and it resembles "the critical spring photoperiodic window" on
intermediate to long days that has been characterized in sheep [37]. Some possible biological mechanisms for our current finding are
briefly discussed in the following. The very long day (20 to 24 hours of daylight) might
challenge the network within the circadian pacemaker that is comprised of the so-called
evening and morning active cells, and that takes part in the seasonal adaptation in
diurnal animals such as fruit flies [38,39] and sheep [37,40-42]. If this holds for humans as well, it is not known at the moment. If it
does, it could mean that, when day lengths approximate fall and winter, the morning
active cells dominate the circadian output, e.g. the sleep-wake behavior. This dominance
of hierarchy is gradually transferred to the evening active cells as the days get longer
in spring [38,39], the coincidence effect of the morning and evening active cells
disappearing when the melatonin signal duration becomes insufficient to sensitize
adenylate cyclase and to support a peak expression of the morning-active cells [37]. Interestingly, the speeding up of the evening
active cells (e.g. by sunshine) makes the morning active cells run faster in long
(summer) but not in short (winter) days [38]. In
Finland, which is located at high to temperate latitudes with the light-dark transitions
being most rapid around equinoxes, the asymmetrical photoperiod possibly favors the
evening-active cells, and produces pronounced melatonin-dependent effects on gene
expression during spring and fall. Whether such "locked morning active cells" contribute
to the peak in deaths from suicide in spring in particular is not known. However, CRY2
and PER2 genetic variants, which might influence the evening and morning signals from
the circadian pacemaker system, associate with depression vulnerability [43,44] in
humans. Therefore, depressed individuals in particular might suffer from entrainment
errors during periods that challenge the circadian pacemaker and predispose to circadian
misalignment.

Local daily weather changes

The complexity of the circadian pacemaker system suggests that signals other than the
seasonal changes in photoperiod, such as temporary variations in local weather
conditions, are likely to play a role in the entrainment process [35,45]. Our finding of the
later suicide peak in the northern area of study, Oulu-region, supports this. However,
the daily mean of suicide mortality was almost as high also during the Y1 period in
Oulu, as in Helsinki.

Hereafter we discuss the potential influence of daily weather changes for the suicide
mortality in Helsinki and Oulu during the peak periods.

During the most dangerous periods, Y1 in Helsinki and X2 in Oulu, days with T+S+ seemed
to be the worst for suicide mortality. From the circadian point of view the long
daylight combined with the daily increase in ambient temperature and sunshine hours
(T+S+) may have further phase advanced the circadian rhythm of the male suicide victims.
An increase in sunshine hours and exposure to light may accelerate and advance the phase
of the principal circadian clock, but an increase in ambient temperature and exposure to
heat may have a similar effect [46]. The peaks of
suicides have associated with ambient temperature in earlier studies [47-49], but so
far, to our knowledge, the role of the circadian clocks has not been addressed.

Many lines of evidence suggest that abnormalities in the thermoregulatory processes are
common among the depressed and therefore may cause or maintain the circadian
misalignment. Patients with a major depressive episode tend to have elevated body
temperature throughout the night, not during the day, and a phase advance of the
circadian rhythm of core body temperature [18].
As hot nights might advance the phase of the circadian clock [50], and nocturnal body temperature during rapid-eye-movement sleep
is influenced by hot, not cold, ambience [51],
the dynamics of nocturnal temperatures might contribute to the advanced and rather fixed
phase positions of circadian rhythms in major depressive episodes. In addition, sleep
abnormalities, characteristically excessive rapid-eye-movement sleep at the cost of
slow-wave sleep [17], are likely to give an
abnormal (accelerating) feedback to the principal circadian pacemaker [26]. Further, during winter the duration of
rapid-eye-movement sleep per night tends to increase [52], giving no support to deceleration and thereby favoring the
desynchronization that may result in lowered mood and the subsequent increase in risk of
suicide.

Daily fluctuations in temperature may play a part in the timing of suicides, either in
combination with the long day length, or possibly also as a separate stressor. Studies
concerning the over-activity in the functions of brown adipose tissue among the
depressed [53] are most interesting in this
respect, since the over-activity of brown adipose tissue may lead to reduced adaptation
to rapid changes in ambient temperature that are typical during spring and fall. Once
being activated, brown adipose tissue does not become quiescent easily [54], and if having been over-activated, it may
through the thermoregulatory defect lead to disruption of the sleep-wake cycle and
appetite control, and lead to early morning awakenings and loss of weight of the
affected individual. Whether this kind of "vernalization failure" characterizes a
suicide process and contributes to a mortality peak year after year is not known, but
needs experimental data for analysis. However, in line with this background, for the Y1
period the daily mean of suicide mortality of men was at its lowest during the days of
T-S- in Helsinki (mean = .124, CI = .106-.142, Table 4) and during the days of T-S+ in Oulu (mean = .140, CI = .090-.190, Table
4), suggesting that T- is a common nominator
for the "safer" weather changes in both locations. T-S+ days were the "safest" also
during the X2 period in Oulu. The daily decrease in temperature could therefore serve as
a protective change during otherwise warm season. However, as the daily means did not
differ significantly between the four types of weather changes, this is somewhat
speculative thus far.

Limitations

Our limitation here is that we did not have the diagnostic information of the suicide
victims, and that we demonstrate associations only, which do not necessarily tell
anything about causality. Another limitation is that we did not have access to a
suitable method, e.g. molecular-timetable methods [55] to be applied to a range of tissues, such as the brain and brown adipose
tissue, from autopsy studies, to be able to analyze a mechanism of action and thereby to
demonstrate a potential link between abnormalities in the circadian pacemaker system and
death from suicide. On the other hand, our strengths include the nationwide sample of
suicides for a long period of time, from a country with a high suicide mortality
rate.

Conclusions

Our main findings here are that suicide mortality is higher during summer months and that
daily changes in sunshine and ambient temperature are likely to modify the suicide
mortality. Our findings presented herein now wait for tests by others in independent
materials and is thus open to replication and the subsequent verification or falsification
of the hypothesis. Some experimental data would be urgently needed for explanation of the
mechanisms of action that take place in the brain of depressed patients and predispose
them to suicide within those particular periods of time that we identified here. Suicide
is a long process, whereas the timing of death from suicide appears far from random. In
Finland from 1969 to2003 suicide mortality was elevated from May 15th to July
25th. This phenomenon should be considered also in clinical practice, since
it bears implications for suicide prevention.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

Authors TP and KS designed the study and wrote the protocol. Author JL conceived and took
part in designing the study. Authors LH and TP managed the literature searches and
analyses and author LH wrote the first draft of the manuscript. All authors contributed to
and approved the final manuscript.

Acknowledgements and funding

We thank Professor (Emeritus) of Mathematics Seppo Mustonen, PhD, University of Helsinki,
Docent of Astronomy Heikki Oja, PhD, Almanac Office at the University of Helsinki, and the
meteorologists Anneli Nordlund and Seppo Sarkkula at the Finnish Meteorological Institute,
all in Helsinki, Finland, for their help in data processing.

The Finnish Cultural Foundation, Finnish National Graduate School for Clinical
Investigation, and Finnish Graduate School of Psychiatry allocated scholarships (to LH)
for this project but had no further role in study design.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License
(http://creativecommons.org/licenses/by/3.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright is retained by the author(s).